Peppermint with the scientific name of Mentha piperita L. is valuable medicinal and aromatic plant from Lamiaceae (Labiatae) family which due to the essential oil in its vegetative body special leaves is a pleasant smell and a cool, slightly spicy (peppery) taste. The average essential oil content of the aerial parts is 1 to 1.5%. This plant has medicinal properties and many usages including anti-flatulent, antibacterial drugs, mouthwash solutions and also has great applications in food, health, cosmetics and soft drink industries. In order to examine the essential oil yield and its quality of peppermint affected by different growth stages, an experiment was performed based on a completely randomized design with five replications at the Special Crop Laboratory, College of Agriculture, Ferdowsi University of Mashhad during year of 2012. Leaves and branches of peppermint were collected at three stages including before flowering, full flowering and after that then dried in shade. Results showed that growth stages had a significant effect on essential oil content of peppermint (p≤0.01). The highest essential oil content for Peppermint was observed in full flowering stage with 1.3%. The main content of essential oil was also influenced by harvest stage; the major essential oil compounds for peppermint at the flowering stage were Menthol, Menthofuran, Menthone with 16.5, 12.1, 8.5%, respectively. Thus, the results suggest that harvest stage should be determined based on their qualitative and quantitative characteristics in order to achieve the highest quality and quantity yield of secondary metabolites in medicinal plants and prevent the loss in essential oil and its quality.

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